This invention relates to a pulse phase control method and apparatus and more particularly to a method and apparatus for controlling the phase of a firing pulse signal applied to a power conversion thyristor or an electrostatic bulb equivalent thereto.
In the case where a dc motor is controlled by a stationary Leonard system, an ac power source and thyristors connected therewith are customarily used and the voltage applied to the dc motor from the power source is controlled by controlling the firing phases of the thyristors. With this type of control system, there is a need for a circuit for generating gating pulses for the thyristors which pulses have a fixed phase relationship according to a control command, with respect to the instant when the voltage of the ac power source crosses zero in its excursion.
U.S. Pat. No. 3,601,674 discloses a method in which the contents of a register for setting data representative of the phase of a firing pulse and the contents of a counter for counting clock pulses having a constant frequency are compared in a digital comparator which in turn generates an output pulse when both the contents coincide with each other. This method, which can change the data at any desired moment, has an advantage that a quick response characteristic can be obtained. On the other hand, Japanese Pat. No. 3452/78 has proposed a method in which after data representative of the phase of a firing pulse has been set in a counter in synchronism with an ac source voltage, the counter counts clock pulses having a constant frequency, so that a firing pulse is generated in response to an overflow pulse delivered by the counter. This method, contrary to the first-mentioned method, can dispense with the digital comparator and therefore is more simple in construction. However, it has a disadvantage that a rapid response is impossible since the data can be changed only once within a cycle of the source voltage.